The screen plates used in conventional digesters for the production of chemical cellulose pulp (e.g. kraft pulp), both continuous and batch digesters, but particularly in continuous digesters, have been a source of downtime and inadvertently introduced non-uniformities in the pulp produced for many years. Despite a wide variety of attempts at particular screen design, and alternating between what screens liquid is withdrawn from (particularly in the extraction stage of a continuous digester, but also in cooking and washing stages), the screens continue to be a source of problems.
In published international application WO 96/26315 (the disclosure of which is hereby incorporated by reference herein) a significant advance in screen plate design was provided by utilizing a plurality of bars, or machined openings in a plate, to provide slots that are disposed at an angle of inclination of between about 30-60 degrees (preferably about 45 degrees) with respect to the horizontal when utilized in a digester. The preset invention provides an improvement in the screen design in WO 96/26315 that even further enhances screen functionality, and ease of construction, while minimizing blockage.
According to one aspect of the present invention a screen used in the production of cellulose pulp in a digester (e.g. a continuous or batch digester, but particularly a continuous digester) is provided comprising: The screen comprising a frame, and a metal screen plate secured to the frame, the screen plate having a plurality of slots formed therein. The screen plate positioned in the digester so that the slots have an inclination angle .alpha. relative to the horizontal or vertical is between about 30-60 degrees. And, wherein a plurality of (substantially horizontally, vertically, and/or diagonally extending) land areas are provided between regions of the slots in the screen plate. Preferably the slots are machined in the screen plate. The screen plate preferably comprises an inner surface facing the inside of the digester and for engaging cellulosic fibrous material slurry in the digester; and an outer surface; and further comprises a deflection minimizing means, preferably a plurality of metal support pins connected to the screen plate outer surface and extending outwardly therefrom. The support pins may have any cross-sectional configuration (e.g. circular, quadrate, triangular, etc.) and typically extend into contact with the digester outer shell so as to provide their support function, and preferably are welded to the screen plate. The metal support pins connected to consecutive land areas are preferably offset from each other, in fact so that in different land areas the pins are disposed on an imaginary straight line that makes an angle of about a with respect to the horizontal or vertical. In the preferred embodiment the angle .alpha. is about 45 degrees.
Preferably the frame is part of the digester, connected to the digester outer shell, and the screen plate is pivotally connected to the frame so that the screen plate may be pivotally moved away from the frame. Typically the screen plate has first and second sides, and is pivotally connected to the frame along a substantially vertical axis at the first side of the screen plate.
In the preferred embodiment, the slots formed on the screen plate taper outwardly at the outer surface (e.g. from the inner surface to the outer surface or from a point with the plate to the outer surface) at an angle .beta. of between about 10-60 degrees, preferably about 20-40 degrees, most preferably about 30 degrees. The digester has a radius at the screen, and the inner and outer surfaces preferably have a radius of curvature approximately equal to the radius of the digester (slightly less than the radius of the out shell).
Preferably substantially all of the slots in a particular screen plate have the same inclination angle .alpha., although it may be with respect to a side edge of the screen plate rather than the top or bottom edge. Alternatively, some (typically a third or less) of the slots in a particular plate may have substantially vertical or horizontal orientations, instead of the angle .alpha..
According to another aspect of the present invention a screen plate per se is provided comprising: A substantially quadrate (e.g. square or rectangular) piece of metal having substantially parallel top and bottom edges, and substantially parallel first and second side edges (substantially perpendicular to the top and bottom edges). A plural slots formed in the piece of metal. The slots having an inclination angle .alpha. relative to one edge of between 30-60 degrees and a width of between about 2-13 mm, preferably about 3-9 mm. And, a plurality of land areas provided between regions of the slots in the piece of metal. The plate is preferably curved and has an are of between about 20-60 degree The details of the screen plate per se preferably are as described above.
It is the primary object of the present invention to provide an enhanced screen structure for use with digesters in the production of chemical cellulose pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.